The present invention relates to an optical disk device for recording data on an optical disk or reproducing it therefrom by causing an optical head to track along the optical disk and, more particularly, to an optical disk device wherein stability of tracking control is improved.
Data retrieval devices have been recently developed and are commercially available which store image data such as documents, which are increasingly handled, on a data file device (e.g., an optical disk recording device) and which retrieve desired image data among various pieces of image data stored in the file device as needed.
In an optical disk device used as the data file device, a document surface to be stored is optically scanned with laser beams and black-and-white data on the document surface is converted to an electric signal (analog signal). The electric signal is then FM modulated to form a record signal. With the record signal, the laser beams are on-off modulated. The modulated laser beams are radiated on an optical disk (rotated at a constant speed) comprising a glass disk and a metal layer deposited thereon. Upon radiation of the laser beams, the metal layer is melted and deformed in accordance with the modulated contents of the laser beams to form pits. Data is stored in the form of a series of pits which form a track with rotation of the optical disk.
For reading out data recorded on the optical disk, the optical disk is rotated at a constant speed. The laser beams are radiated along a data recording track on the rotating disk for scanning. As a result, modulated reflected light beams are obtained in accordance with the arrangement of the pits formed on the optical disk. The reflected light beams are photoelectrically converted to reproduce data recorded on the disk.
In the optical disk device of this type, for recording data on the optical disk or reproducing it therefrom, the data recording tracks on the rotating optical disk must be properly traced and scanned with the laser beams. The following device is used to trace the data recording tracks.
The device has a photoelectric transducer having a pair of light-receiving elements arranged along the radial direction of the optical disk. Two outputs from the photoelectric transducer are compared in a differential amplifier to obtain a tracking error signal. The tracking error signal is divided into a high frequency component and a low frequency component by a servo circuit. The servo circuit drives a linear motor in accordance with the low frequency component and drives a galvanomirror in accordance with the high frequency component. Thus, tracking is performed.
However, the servo circuit described above requires two servo loops, the stability of which is liable to be affected by the amount of eccentric displacement. Further, the arrangement of the servo circuit is complex.